The present invention pertains to a process for recovering magnesium oxide and nickel from magnesium-bearing materials, and more particularly, to a process for recovering nickel and producing high purity, high surface area magnesium oxide (MgO) suitable for forming products for refractory applications.
Magnesium oxide is useful in the formation of materials for refractory applications, i.e., materials suitable for use at high temperatures, such as bricks for lining steel-making furnaces. Refractory magnesium oxide is formed by dead burning magnesium oxide at temperatures of about 2100.degree. C. after briquetting at pressures exceeding 20,000 psi. It is desirable that the magnesium oxide feed for the briquetting and dead burning processes have both high purity and high surface area. In the prior art, additives have been used to render the MgO suitable for successful briquetting and densification. Because of the additives required, refractory MgO briquettes having the desired density and purity could not easily be formed.
High purity MgO is rare in nature. Naturally occurring magnesite suitable for refractory applications is costly and is becoming increasingly difficult to obtain.
At the same time, magnesium-containing materials of lesser purity abound. Magnesium is found in laterites, brines, sea water, dolomite, and many other materials. However, magnesium oxide is difficult to obtain in high purity from these materials because of the presence of other metals and impurities of similar properties, making physical or chemical separation difficult.
The prior art teaches the leaching of magnesium-containing ores with sulfuric acid to solubilize the metals, including magnesium, as sulfates. The metal sulfates solution is then subjected to various physical and chemical treatments to separate and purify the desired magnesium oxide. One prior art process, disclosed in U.S. Pat. No. 4,096,235, teaches neutralization of the metal sulfates solution with magnesia to precipitate impurities, such as iron, as hydroxides. The magnesium sulfate remaining is crystallized, dried and decomposed in a fluidized bed reactor to form solid magnesium oxide and sulfur dioxide gas. The SO.sub.2 is then recycled to form H.sub.2 SO.sub.4. However, the prior art has not considered the sulfate system suitable for the commercial production of refractory MgO.
One of the difficulties with this and other of the prior art processes is the necessity of utilizing starting materials of relatively high magnesium content, such as magnesite. This limitation results from the fact that the neutralization step which precipitates metals other than magnesium as hydroxides fails to remove significant amounts of borates, iron, calcium and silicates. Though lower purity starting materials can be used, the resulting lower purity MgO is less desirable for refractory applications. To achieve higher purity MgO from such starting materials, additional chemical purification and crystallization stages are required, greatly increasing the cost of the product.
Because of the low surface area characteristic of the MgO formed by the prior art processes, additives to the MgO are generally required prior to dead burning to allow successful briquetting and densification. Such additives increase the cost, and of course, decrease the purity of the resulting refractory material. The prior art does not teach a method for economically and consistently producing MgO of controlled surface area from MgSO.sub.4.
Many magnesium containing ores, such as Oregon laterites, additionally contain significant amounts of nickel and iron. The recovery of iron and nickel from these ores has not heretofore been commercially practicable. However, if such recovery were possible, it could greatly enhance the profitability of refining such ores. Additionally, it is desirable that the more valuable nickel be recovered in a higher ratio relative to the iron than the naturally-occurring ratio of these elements in the unprocessed ore.